The invention relates generally to the isomerization of hydrocarbons. More specifically, the invention involves an isomerization zone followed by adsorptive separation of the isomerate into an extract stream and a raffinate stream followed by fractionation of the extract and of the raffinate to recover desorbent and finally further fractionation of the raffinate in a dividing wall fractional distillation column.
In many commercially important petrochemical and petroleum industry processes it is desired to separate closely boiling chemical compounds or to perform a separation of chemical compounds by structural class. It is very difficult or impossible to do this by conventional fractional distillation due to the requirement for numerous fractionation columns which may consume excessive amounts of energy. The relevant industries have responded to this problem by utilizing other separatory methods which are capable of performing a separation based upon chemical structure or characteristics. Adsorptive separation is one such method and is widely used to perform these separations.
In the practice of adsorptive separation a feed mixture comprising two or more compounds of different skeletal structure is passed through one or more beds of an adsorbent which selectively adsorbs a compound of one skeletal structure while permitting other components of the feed stream to pass through the adsorption zone in an unchanged condition. The flow of the feed through the adsorbent bed is stopped and the adsorption zone is then flushed to remove nonadsorbed materials surrounding the adsorbent. Thereafter the desired compound is desorbed from the adsorbent by passing a desorbent stream through the adsorbent bed. The desorbent material is commonly also used to flush nonadsorbed materials from the void spaces around and within the adsorbent. This could be performed in a single large bed of adsorbent or in several parallel beds on a swing bed basis. However, it has been found that simulated moving bed adsorptive separation provides several advantages such as high purity and recovery. Therefore, many commercial scale petrochemical separations especially for specific paraffins and xylenes are performed using simulated countercurrent moving bed (SMB) technology.
The passage of the desorbent through the adsorbent dislodges the selectively retained compounds to produce an extract stream. The extract stream contains a mixture of desorbent and the desorbed compounds, with these materials being then separated by distillation in a column referred to as the extract column. The raffinate stream contains a mixture of desorbent and the non-adsorbed compounds, with the desorbent being removed from the raffinate stream by distillation in a column referred to as the raffinate column. The subject invention is aimed at improving the ultimate product of the isomerization process through improving the fractionation employed in recovering the final desired compounds from the raffinate stream.
Several economic advantages are derived from the continuous, as compared to batch-wise, operation of large-scale adsorptive separation processes. Recognition of this has driven the development of simulated moving bed (SMB) adsorptive separation processes. These processes typically employ a rotary valve and a plurality of lines to simulate the countercurrent movement of an adsorbent bed through adsorption and desorption zones. This is depicted, for instance, in U.S. Pat. No. 3,205,166 to D. M. Ludlow, et al. and U.S. Pat. No. 3,201,491 to L. O. Stine et al.
U.S. Pat. No. 3,510,423 to R. W. Neuzil et al. provides a depiction of the customary manner of handling the raffinate and extract streams removed from an SMB process, with the desorbent being recovered, combined and recycled to the adsorption zone. U.S. Pat. No. 4,036,745 describes the use of dual desorbents with a single adsorption zone to provide a higher purity paraffin extract. U.S. Pat. No. 4,006,197 to H. J. Bieser extends this teaching on desorbent recycling to three component desorbent mixtures.
The dividing wall or Petyluk configuration for fractionation columns was initially introduced some 50 years ago by Petyluk et al. A recent commercialization of a fractionation column employing this technique prompted more recent investigations as described in the article appearing at page s14 of a Supplement to The Chemical Engineer, Aug. 27, 1992.
The use of dividing wall columns in the separation of hydrocarbons is also described in the patent literature. For instance, U.S. Pat. No. 2,471,134 issued to R. O. Wright describes the use of a dividing wall column in the separation of light hydrocarbons ranging from methane to butane. U.S. Pat. No. 4,230,533 issued to V. A. Giroux describes a control system for a dividing wall column and illustrates the use of the claimed invention in the separation of aromatics comprising benzene, toluene and orthoxylene.
The use of the dividing wall column in the present invention is a significant advantage over isomerization flow schemes that do not employ a dividing wall fractionation column, such as that described in Domergue, B., Watripont, L. World Refining, May 2000, p. 26-30.
One purpose of the invention is to provide a method for separating 2-methylpentane and 3-methylpentane from a mixture containing at least 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylpentane, isopentane, methylcyclopentane, cyclohexane, and C7+ hydrocarbons. The mixture is introduced into a dividing wall fractionation column operated at fractionation conditions and divided into at least a first and a second parallel fractionation zone by a dividing wall, with the first and the second fractionation zones each having an upper and a lower end located within the fractionation column, with the first and second fractionation zones being in open communication at their upper ends with an undivided upper section of the fractionation column and in open communication at their lower ends with an undivided lower section of the fractionation column. The mixture is introduced to the column at an intermediate point of the first fractionation zone. A stream of 2-methylpentane and 3-methylpentane is removed from an intermediate point of the second fractionation zone of the dividing wall fractionation column; a stream of, 2,2-dimethylbutane, 2,3-dimethylpentane, and isopentane, is removed from a first end of the dividing wall fractionation column, and a stream of methylcyclopentane, cyclohexane, and C7+ hydrocarbons, is removed from a second end of the dividing wall fractionation column.
A specific embodiment of the invention provides an isomerization process having an adsorptive separation zone and a dividing wall fractionation zone. A feed stream containing at least normal pentane and normal hexane is contacted in an isomerization zone with an isomerization catalyst under isomerization conditions to convert at least a portion of the normal pentane and normal hexane into isomerized products and form an isomerization zone effluent containing normal pentane, normal hexane and isomerized products. This effluent is passed to an adsorptive separation zone comprising a bed of a selective adsorbent maintained at adsorption conditions under which the normal pentane and normal hexane are selectively retained on a quantity of the selective adsorbent, thus forming an isomerized product stream containing at least the isomerized products. The normal pentane and normal hexane are desorbed under desorption conditions to yield a normal alkane stream of at least normal pentane and normal hexane. The isomerized product stream is passed into a dividing wall fractionation column operated at fractionation conditions and divided into at least a first and a second parallel fractionation zone by a dividing wall, with the first and the second fractionation zones each having an upper and a lower end located within the fractionation column, with the first and second fractionation zones being in open communication at their upper ends with an undivided upper section of the fractionation column and in open communication at their lower ends with an undivided lower section of the fractionation column. The isomerized product stream is introduced at an intermediate point of the first fractionation zone. A low-octane stream of 2-methylpentane and 3-methylpentane is removed from an intermediate point of the second fractionation zone of the dividing wall fractionation column. A first high-octane stream is removed from a first end of the dividing wall fractionation column, and a second high-octane stream is removed from a second end of the dividing wall fractionation column.
Another specific embodiment of the invention provides an isomerization process having an adsorptive separation zone, an integrated fractionation zone, and a dividing wall fractionation zone. A feed stream containing at least normal pentane and normal hexane is contacted with an isomerization catalyst under isomerization conditions in an isomerization zone to convert at least a portion of the normal pentane and normal hexane into isomerized products and form an isomerization zone effluent containing normal pentane, normal hexane and isomerized products. The isomerization zone effluent is passed to an adsorptive separation zone having a bed of a selective adsorbent maintained at adsorption conditions under which the normal pentane and normal hexane are selectively retained on a quantity of the selective adsorbent, thus forming a raffinate stream of the isomerized products and desorbent formerly present in the quantity of the selective adsorbent. Desorbent is contacted with the quantity of the selective adsorbent which has retained the normal pentane and normal hexane under desorption conditions to yield an extract stream of normal pentane, normal hexane, and the desorbent. The extract stream is passed to an integrated fractionation column operated at fractionation conditions and divided into at least a first and a second vertical fractionation zone, with each zone having an upper and a lower end located within the fractionation column, with the first and second fractionation zones being in open communication at their upper ends at a first end of the column and with the extract stream entering the fractionation column at an intermediate point of the first fractionation zone. The raffinate stream is passed into an intermediate point of the second fractionation zone of the integrated fractionation column. An extract product stream is removed from a first end of the first fractionation zone, with the first end not being in communication with the second fractionation zone and being located at the second end of the integrated fractionation column. A raffinate product stream is removed from a first end of the second fractionation zone, with the first end not being in communication with the first fractionation zone and being located at the second end of the integrated fractionation column. A desorbent stream is removed from the first end of the integrated fractionation column. The raffinate product stream is passed to a dividing wall fractionation column operated at fractionation conditions and divided into at least two parallel fractionation zones, Zone A and Zone B, by a dividing wall, with Zone A and Zone B each having an upper and a lower end located within the fractionation column, with Zone A and Zone B being in open communication at their upper ends with an undivided upper section of the fractionation column and in open communication at their lower ends with an undivided lower section of the fractionation column, and with the raffinate product stream entering the column at an intermediate point of Zone A. A lowoctane stream of 2-methylpentane and 3-methylpentane is removed from an intermediate point of Zone B of the dividing wall fractionation column. A first high-octane stream is removed from a first end of the dividing wall fractionation column, and a second high-octane stream is removed from a second end of the dividing wall fractionation column.